The demand for lithium secondary batteries to meet high power and high-energy system applications has resulted in substantial research and development activities to improve their safety, as well as performance. As the world becomes increasingly dependent on portable electronic devices, and looks toward increased use of electrochemical storage devices for vehicles, power distribution load leveling and the like, it is increasingly important that the safety of such devices be paramount, especially as these devices are being used in such environments as airliners and space vehicles. The effort to date has included research in flame-retardants, solid polymer electrolytes and new electrolyte concepts with improved thermostability. Thus, the development of highly conductive electrolytes, free of any problems associated with volatile and combustible solvents, is of paramount importance. Electrolytes based on polymeric structures have basically better heat and chemical resistance than conventional organic carbonate-based electrolytes and can thus reduce many chemical side reactions occurring in lithium secondary batteries. Although, polymeric based electrolytes have many advantages over carbonate solvent based electrolytes, their application in lithium secondary batteries has been limited due to their low ionic conductivity, usually below 10−5 S/cm at room temperature.
To solve this problem, new electrolyte concepts are needed. The new electrolytes should be nonvolatile materials that have excellent electrochemical properties, such as high ionic conductivity of over 10−4 S/cm at room temperature and wide electrochemical stability windows of over 4.5 V (based on lithium metal).
Accordingly, the present inventors have developed a new type of ionically conductive electrolyte based on various polymeric structures, especially having a poly(siloxane-g-ethylene oxide) composition which overcomes the above mentioned problems of volatility, flammability and chemical reactivity inside of the lithium battery. The proposed liquid type poly(siloxane-g-ethylene oxide) materials also have an excellent electrochemical stability window and favorable room temperature ionic conductivity.
Carbonate solvents, such as ethylene carbonate and ethyl-methyl carbonate used for conventional lithium battery electrolytes can easily bum from low temperature sources of ignition and generate flame, carbon dioxide and H2O during thermal degradation. This is a critical problem in high capacity battery applications like lithium batteries for electric vehicles (EV) and satellites. Polymeric materials, however, usually have a somewhat different combustion mechanism than the carbonates. The initial stage of a fire occurs when a heat source decomposes the polymeric materials to flammable volatile products. Thus, for continuous burning to occur, (a) the application of heat must be sufficient to decompose the polymer, (b) the temperature must be sufficient to ignite the products of decomposition, and (c) the amount of heat transferred from the flame back to the polymer must be sufficient to maintain the cycle. In general, polymeric materials are more thermally stable than low molecular weight chemicals like the organic carbonates because they are not volatile and are vaporized at much higher temperatures.
The present inventors have carefully considered the combustion mechanisms of polymeric materials and concluded that to stop the propagation of the burning cycle one needed to develop new polymer materials that are thermally more stable and capable of dissolving the lithium salts to prepare electrolytes for electrochemical devices such as lithium batteries and/or capacitors. The present inventors have developed new structural siloxane polymers with one or more poly(ethylene oxide) side chains. Siloxanes are very thermally stable and are decomposed by heat with difficulty. Only a few flammable by-products are formed during the thermal decomposition of such polymers because their main chain is a Si—O linkage. Thus, its presence in the proposed polymers will delay the initiation of the combustion cycle.
Due to the merits of siloxane-poly(ethylene oxide) graft copolymers, substantial research has been done. See, for example, U.S. Pat. No. 5,112,512 to Nakamura and U.S. Pat. No. 6,124,062 to Horie et al. also describing siloxane-poly(ethylene oxide) graft copolymers (as a polymeric electrolyte material. The '512 patent discloses a crosslinked polymer electrolyte based on the graft copolymers, but its ionic conductivity is too low for room temperature applications. The '062 patent discloses direct use of siloxane-poly(ethylene oxide) grafted copolymers as a liquid for a lithium battery electrolyte with ionic conductivity of around 10 −4 S/cm at ca. 25° C. (See General Formula I). The conductivity of the material disclosed in the '062 patent is disadvantageously low. In addition, the cost of such material is relatively high. The present inventors have discovered a much improved material with higher conductivity and lower cost.
General formula (I) (as disclosed in the '062 patent):
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wherein R is alkyl group.
The present inventors developed an improved liquid state siloxane polymer with one or more poly(ethylene oxide) side chains to ensure thermal stability and electrochemical properties. Poly(ethylene oxides) (PEO) in poly(siloxane-g-ethylene oxide) materials of this invention (see general formula II) are directly bonded to a Si atom. Poly(siloxane-g-ethylene oxide) materials of the present invention are easily synthesized through a simple dehydrocoupling reaction with simple metal carbonate based catalysts and the cost for synthesis of poly(siloxane-g-ethylene oxide) of this invention is much lower than the cost of synthesis of general formula (I) with a propylene spacer between siloxane and PEO. The present inventors also control the viscosity of the materials to get high ionic conductivities of around 10−3 S/cm at room temperature.
where R and R″ are alkyl groups and R′ is hydrogen or alkyl group.
As compared to the material disclosed in the '062 patent, ionic conductivity is improved by changing the chemical structure of siloxane, that is, directly grafting ethylene oxide onto Si atom without any alkyl carbon spacer between them. Changing the structure in this way increases hydrophilicity and solubility, leading to higher conductivity.